Light scattering has been a problem during the development of light emitting diode on silicon (LEDoS) device. It is due to the fact that the high refractive index of sapphire substrate of the LED chip enhance scattering. In addition, the use of phosphor particles for light conversion from blue to green and from blue to red colors will further enhance the scattering. This problem leads to unclear images when the LED pointed-light source is used to project light onto a screen, and adversely affects the LEDoS to be used for projector application.
Insertion of collimator or grid may help to reduce scattering, as demonstrated in U.S Patent Application Publication No. 2012/0223875, but the light intensity is also reduced since the grid/collimator absorbs a certain amount of light. Similarly, it is still not favorable when the light source is used for projector application. '875 is also not suitable for LED array in relatively smaller size as their claimed device lowers the light intensity of the light travelling through the grid-like structure due to the size limitation.
The problem becomes worse when the size of micro-display is getting smaller. For a collimator with channel size less than 100 μm, the light absorbed by the sidewall becomes significant and the intensity of emitted light is reduced dramatically.